Function for distance along irregular (loop) path

I'm trying to make a reinforcement learning driving simulator, and I need a fitness function which will tell me which cars perform the best (so I can mutate them and form the next generation). If the track is a perfect circle I can use tan(y/x), and this will unevenly work for any convex shape, but what if the track is concave? Can I use tan() if I define it as a series of arcs? Is there no such function?